Operation of chlor-alkali cells



'.J. E. CURREY ETAL OPERATION OF' CHLOE- ALKALI CELLS Sept. 24, 1968 `5Sheds-Sheet l Filed NOV. 29, 1965 sept. 24, 196s J E, CURREY- TAL3,403,083

OPERATION OF CHLORALKALI CELLS Filed Nov. 29, 1965 5 Shees-Shee(l 2ODG-J .Emu Z :Coz mmh: mmm mwm@ 0mm :@WN ONN Oom ow.

5 Sheets-Sheet 5 J. E. cuvRRr-:Y ETAL OPERATON OF CHLOR-ALKALI CELLS lsept. 24, 1968 Filed NOV. 29 1965 @Nm oo. n

United States Patent O 3,403,083 OPERATION F CHLOR-ALKALI `CELLS John E.Currey, Niagara Falls, N.Y., John Rutherford, Luliug, La., Dudley P.Fernandes, Montague, Mich., and Robert J. Leonard, Niagara Falls, N.Y.,assignors to Hooker lChemical Corporation, Niagara Falls, N.Y., acorporation of New York Filed Nov. 29, 1965, Ser. No. 510,225 20 Claims.(Cl. 204-98) This invention relates to chlor-alkali diaphragm cells andmore particularly to a method of operating chloralkali diaphragm cellsalone or in groups under controlled conditions of pH, anolytesaltconcentration and temperature, to thereby increase the efficiency ofsuch cells, to control the proportions of chlorine and caustic sodaproduced, to reduce anode consumption, to produce a cell liquor ofincreased caustic soda content and to effect numerous other improvementsin cell operations, as hereinafter disclosed.

Chlor-alkali diaphragm cells have for years been run in large groups of50 to 100 or more cells, each cell running independently of the othercells. The products from these cells were combined into three majoreflluent streams so that the total groups of cells produce an etiiuentstream of chlorine, an eiiluent stream of hydrogen and an effluentstream of cell liquor. The cell liquor was a mixture of about 9 to 12weight percent caustic and about 10 to 18 weight percent salt.

With each cell running individually, only very limited control could bemaintained on the operation of the cell such as could be achievedthrough changes in the decomposition voltage Iand regulation of thebrine concentration and feed rates which in turn, depended largely onthe porosity and flow rate through the diaphragm. This resulted in eachcell operating at different eiciencies and under different conditions oftemperature, anolyte concentration, pH, and so forth. As an example,cells having new anodes maintain an operating temperature which is lowerthan the most etlicient operating temperature. In turn, cells havingold, worn anodes tend to operate at ternperatures above the mostefficient operating temperature and often result in the expenditure ofexcessive amounts of current in heating the electrolyte to its boilingpoint.

The major items of 'expense in operating diaphragm cells are (l) power,(2) cell renewal and (3) caustic evaporation. All of these items aredirectly related to the cell operation in (1) 'current efliciency, (2)cell voltage, (3) anode life, (4) diaphragm life and (5) cell liquorcaustic and salt concentration. If the current efficiency and cellvoltage could be advantageously controlled to reduce power costs andextend the anode life, and if the diaphragm life could also be extended,two of the major cost items in operating diaphragm cells would bereduced. Further, if in advantageously controlling the first fourfactors enumerated above, the cell liquor caustic and saltconcentrations could be changed to a more advantageous ratio, then allthree of the major cost items in operating diaphragm cells could bereduced, thereby greatly improving chlor-alkali diaphragm celloperations.

It is an object `of this invention to provide a method whereby thecurrent efficiency of Chlor-alkali diaphragm cells is improved. It isanother object of this invention to provide a method wherebychlor-alkali cell voltages are advantageously improved to -operate atthe most efficient level. A further object of this invention is toprovide a method whereby the anode life of Chlor-alkali diaphragm cellsis extended. Yet, another object of this invention is to provide amethod of operation whereby the porosity of the diaphragm becomes lesscritical, so that the diaphragm life can be extended to equal the anodelife. Another -object of this invention is to improve the caustic-saltcon- 3,403,083 Patented Sept. 24, 1968 centration ratio in the cellliquor by producing a higher concentration of caustic in the cellliquor. These and other `objects will become apparent to those skilledin the art from the description of the invention which follows.

In accordance with the invention, a process is provided for operating agroup of Chlor-alkali diaphragm `cells comprising imposing adecomposition volt-age across the electrodes of a group of Chlor-alkalidiaphragm cells, feeding a solution of brine to the anolyte compartmentsof each of said cells at a rate in excess of the amount which owsthrough the diaphragm of said cells, withdrawing the excess brine feedsolution from the anolyte compartments of said cells, combining andreplenishing the withdrawn solutions with additional amounts of salt andreturning the replenished anolyte brine solution to the anolytecompartments -of said cells. Preferably, the feed brine is a saturatedor nearly saturated solution. In this anolyte recirculation method, thefeed rate of brine to the anolyte compartment is greater than the amountwhich flows through the diaphragm and preferably is in an amount greaterthan one time and up to about ten times the amount which ows through thediaphragm into the catholyte compartment. In addition to operating aseries of cells with the brine feed in parallel, the process can also bereadily effected with each cell on an individual basis. Even further,improvements are obtained wherein temperature and pH adjustments, as bythe addition of acid, are made with the anolyte solution in the cell.

The anolyte recirculation method of the present invention providesgreatly improved control of the anolyte liquor to thereby provide thehighest operating etiiciencies in Chlor-alkali diaphragm cells. Theprocess can be operated with a group 4of cells, thereby establishingcontrolled conditions of temperature, anolyte pH, anolyte and cellliquor concentration, and the like, which are substantially the same inall of the cells, thereby effectively operating all or nearly all of thecells under the most desirable conditions.

The process of the present invention can -be used in the electrolysis ofany alkali metal chloride. However, because sodium chloride is preferredand is normally the alkali metal chloride used, the descriptionhereinafter is directed more particularly to sodium chloride. It is tobe understood, however, that other alkali metal chlorides can be used,particularly potassium and lithium chlorides.

The invention will be further described by reference to the drawings inwhich:

FIG. 1 is a partially schematic flow sheet illustrating the process ofthe present invention, particularly as it relates to operation of agroup of cells;

FIG. 2 is a vertical, partial section of a typical chloralkali cellmodified in accordance with the present invention;

FIG. 3 is a side elevation of FIG. 2 further showing a typicalmodification of the Chlor-alkali cell; and

FIG. 4 is a graph showing the average 4relationship between cell liquorstrength, current efliciency and rate lof HC1 addition for chlor-alkalidiaphragm cells operated in accordance with the present invention.

It has been found that the anolyte salt concentration has an effect onthe current efficiency. Normally, increasing the chloride concentrationin the anolyte compartment results in benefits of higher currentefficiency, purer chlorine, lower voltage, lower graphite consumption,higher caustic concentration and less chlorate in the cell liquor sothat it is normally preferred to operate at the highest saltconcentration. However, the solubility of sodium chloride in the feedbrine limits the amount of sodium chloride which can be practicably fedto a normally operated cell.

Since both chlorine and sodium ions are being removed from the cell atthe electrodes, the bulk anolyte solution becomes depleted of salt tosuch an extent that a normal cell has an anolyte salt concentrationconsiderably below the saturation point, even though the ybrine was fedto the cell as a nearly saturated solution. Typically, a normaldiaphragm cell was fed with a brine solution containing about 320 to 330grams per liter of sodium chloride (a saturated solution at about 90degrees centigrade contains about 333 grams per liter) but has ananolyte sodium chloride concentration of only about 270 grams per liter.Better cell operation is obtainable at higher anolyte saltconcentrations.

It has now been found that by the present anolyte recirculation method,the advantages of higher sodium chloride concentration in the anolytecompartment can be realized by feeding a stream of Ibrine to the anolytecompartment at a rate faster than the brine can pass through thediaphragm into the catholyte compartment. Thus, it is possible toincrease the sodium chloride concentration in the anolyte compartment bycontinuously adding concentrated brine and removing depleted anolytesolution. The excess brine is withdrawn from the anolyte compartment andis resaturated with sodium chloride. Also, it is preferred to adjust thetemperature to the desired operating range and to add H-Cl in an amountto obtain the desired anolyte pH prior to returning the brine to theanolyte compartment. The practical over-all result is that a levelingeffect is obtained in all of the cells and the sodium chlorideconcentration in the anolyte compartment is increased to a level higherthan that previously obtainable. By the present method, the sodiumchloride concentration in the anolyte compartment can be maintained atany level up to the saturation concentration and particularly may bemaintained within the preferred range of 260 to 330 grams per liter ofsodium chloride. Because the recirculation of the anolyte liquor withoutan enrichment of sodium chloride results in substantial improvements inthe operation of Chlor-alkali cells, the present invention can also beoperated with lower NaCl concentrations, such as about l30 to 260 gramsper liter of NaCl.

The process of the present invention is effected, as illustrated by FIG.l, by feeding a concentrated stream of feed liquor to a group ofchlor-alkali diaphragm cells 16 `by means of lateral feed lines 12, 13,14 and 15. The group or series of cells may be 2 to 100 or more cellsfrom which anolyte liquors are Withdrawn and combined for recirculation.The feed rate of brine to the cells 16 by means of the lateral lines 12,13, 14 and 15, is at a rategreater than the amount of liquor which flowsfrom the anolyte compartment through the diaphragm in the chlor-alkalicell into the catholyte compartment and more preferably, the brine feedrate is 1.5 times up to about ten times the flow through the diaphragm.The most preferred flow rate averages, for a group of cells, is abouttwo to five times that flowing through the diaphragm. The excess feedliquor is withdrawn from the cells 16 via lines 18, 20, 22 and 24. Theselines are combined and returned to salt saturator 28 via line 26. Cellliquor 40 is withdrawn from the catholyte compartments of the cells vialines 35, 36, 37 and 38 using suitable withdrawal means.

In salt saturator 28, additional sodium chloride 30 and Water 31, ormixtures thereof, are mixed with the brine to resaturate it with sodiumchloride prior to returning the brine to the anolyte compartments of thecells 16 via line 10. Normal salt saturation techniques are used in thesalt saturation step. In addition to resaturating the withdrawn anolyteliquor, sufficient additional brine is prepared or mixed with theanolyte liquor to replace brine which passes through the diaphragm inthe electrolytic cell.

Another variable `which affects the anode current efficiency is theanolyte temperature. The provisions for heat exchange means 32associated with brine saturator 28, as shown, or other heating means,'provide for maintaining the cells at the most eficient operatingtemperatures. Heat exchange means 32 maintains the vsaturator 28 at theproper temperature for saturating the recirculating anolyte solution sothat the maximum practical salt concentration is supplied to the cells.Normally, saturated brine fed to `the cells contains about 26 to.27percent NaCl by .weight or about 327 grams per liter'of NaCl, which isthe saturation concentration at about 65 degrees centigrade. Additionalheat is provided after the saturator to superheat the brine to atemperature of approximately to 80 degrees centigrade to prevent thedeposition of salt crystals in the feed lines to the cells. 'This lattertemperature (superheat) is regulated so that the temperature of theanolyte in the cell is maintained between about and 100 degreescentigrade by the additional heat provided by the electrochemicalreaction taking place in the cell.

Alternatively, the saturator can be operated at a higher temperature,such as 75 to 8O degrees centigrade, and a small stream of unsaturatedbrine or water may be added after the saturator to reduce the saltconcentration in the brine to about 327 grams per liter to prevent saltdrop out in the lines to cell. Again it is preferred that thetemperature of the saturated brine to cell Ibe regulated so that theoperating temperature in the anolyte compartment of the cell ismaintained at the most preferred temperature of about 93 to 100' degreescentigrade.

In an operating group of electrolytic cells, the amount of heat requiredby heat exchanger 32 varies primarily with the requirements to heat theadditional water or brine added in the salt saturator 28. The heating ofbrine prior to feeding it to the cell is not in itself, new. However,the effect of rapid anolyte turnover and the mixing of the anolyteeffluents from a group of cells produces a cumulative heat exchangeeffect which results in all of the cells operating at more efficienttemperatures independent of the cell age, electrode decomposition,particular cell characteristics and the like factors which previouslydictated the individual cell operating temperature.

As a result of the changes effected in the anolyte compartment by meansof the present anolyte recirculation method, further beneficial changesresult in the entire cell operation. It was previously known that theaddition of hydrochloric acid to a chlor-alkali diaphragm cell having aporous asbestos diaphragm resulted in a tightening of the diaphragm anda restriction of liquid flow through the diaphragm when the anolyte pHdropped to too low a level. When this occurred, the liquid level in theanolyte chamber increased and often the cell would have to be removedfrom service due to the high level. Because of the variations inporosity of deposited diaphragms and the changes effected by acidadditions, the flow rates through the diaphragms varied with each cellsuch that previously the brine feed had to be individually controlled ineach cell to maintain the desired cell liquor strength to compensate forthe added acid. To regulate the anolyte pH Within the most desirable pHrange of 2 to about 4 while maintaining a proper cell level was indeed,a difficult task. To further complicate the matter, changes in the flowthrough the diaphragm affect the back migration of hydroxyl ions whichchanges the acid requirement for each cell. Thus, as a practical matter,large acid additions have not previously been feasible in large scaleoperations.

The present method of anolyte recirculation substantially reduces theneed for individual cell attention due to changes in diaphragm porosity,hydroxyl ion back migration, and the like. The rapid anolyte turnover orflow rate produces a leveling effect in all the cells, whereby thedesired pH range is maintained independent of the particular porosity ofthe diaphragm and back migration. In addition, the effects of arestricted diaphragm are of lesser importance because the anolyterecirculation method maintains the same anolyte liquid level independentof the flow through the diaphragm.

In a preferred embodiment of the present invention, hydrogen chloride 34is added to the saturated or nearly saturated brine withdrawn from saltsaturator 28 via line 10. When HC1 is added, it is added in an amountsuflicient to maintain an anolyte pH within the range of about 0.2 toabout 4.5 and more preferably about 1.5 to 4. The most preferred pHrange is about 2.0 to 3.0. The lowest pH- values are best used with adiaphragm material other than asbestos, such as chlorinated polyvinylchloride, polypropylene, and the like. The amount of HC1 required forthis adjustment varies with the particular operating conditions and canbe in amount up to about 20 percent HCl based on the amount of chlorineliberated at the anode; that is, 20 percent of the chlorine produced isfrom the HC1 addition. With greater amounts of HC1 being added, the pHof the brine fed to the cell can be as low as about 0.2. When no HCl isadded, the pH of the brine fed to the cell is as high as about 7,because the recirculated anolyte lowers the brine pH from the normallyalkaline pH o-f about 9 to a neutral or slightly acidic pH. The HC1added can be added either as a gas or as an aqueous solution.

The pH of the anolyte has been found to be important in establishinghigh current efliciencies in the cell, and especially in attempting toimprove the efliciency of already highly eiiicient cells. In normal celloperations, the back migration of the hydroxyl ions into the anolyteresults in an increase in the anolyte pH while the chlorine evolvedtherein lowers the pH. Cells running individually will vary widely inanolyte pH. Normally, a low anolyte pH is obtained in cells with newdiaphragms and a high anolyte pH is found in cells with olderdiaphragms. As the mechanism of back migration of hydroxyl ions ispresently understood, the migration increases for any particulardiaphragm as the concentration of caustic in the catholyte cell liquorincreases. In turn, the concentration of caustic in the catholyte cellliquor increases because of a decrease in the ow of brine into thecatholyte chamber as a result of a decrease in the porosity of thediaphragm. The decrease in the `diaphragm porosity results from thedeposition of calcium and magnesium compounds and other substances inthe diaphragm pores during use. Thus, over-all brine quality and thenature of the diaphragm are factors which bear significantly on thechanges in anolyte pH and its attendant lower cell elliciency.

In normal cell operation, when the anolyte pH ncreases for any reason,there is no built-in compensating effect to keep it at its proper value.The present invention provides the means for maintaining anolyte pHwithin the desired range by (1) recirculating the anolyte from a groupof cells to obtain the cumulative effect of the anolyte pH of all of thecells so as to result in the cells operating at a pH which is theaverage thereof and/or by (2) the addition of HC1 to the brine feed,with anolyte recirculation. Thus, the cell can always be kept operatingat the most effective pH for peak elliciency substantially independentlyof the porosity of the diaphragm and the concentration of the caustic inthe catholyte chamber.

Being able to use a tight diaphragm or a diaphragm of lower porosity hasthe added benefit of enabling cell operation at higher causticconcentrations in the catholyte compartment. Whereas previously thecatholyte solution (cell liquor) contained about 9 to 12 percent sodiumhydroxide, caustic concentrations can now be in the range of 12 to about22 percent in the catholyte cell liquor or about 145 to 270 grams perliter of NaOH, while the desired anolyte pH is maintained. As Will bereadily realized, the process can also be operated to obtain normal cellliquor strengths of about 110 to 150 grams per liter of NaOH. Bycontrolling the anolyte pH at the desired elicient operating level, suchas by increasing the recirculation rate and/or using brinewith enoughHC1 dissolved in it to compensate for increased back migration of thehydroxyl ion, the former limiting factor of the hydroxyl back migrationis mitigated. By operating the cells to increase the causticconcentration in the catholyte compartment a higher ratio of caustic tosodium chloride in the cell liquor is obtained. Thus, over twice thenormal caustic concentration can be obtained in the cell liquor, therebygreatly reducing the evaporation and concentration costs normallyotherwise incurred.

FIG. 2 and FIG. 3 illustrate a Chlor-alkali diaphragm cell modified soas to utilize the present anolyte recirculation method. A typical cell44 having an anolyte compartment 46 separated from a catholytecompartment 48 by means of a porous diaphragm 50 is used. Catholytecompartment 48 has an overflow means 49 by which cell liquor iswithdrawn from the cell. Within the anolyte compartment 46 are brinefeed means` 52, chlorine gas removal means 54, anodes 56 and anolyteliquid Withdrawal means 64. Attached to the anolyte compartment is sightglass 60 which shows the level of anolyte liquor within the anolytecompartment.

Anolyte liquor withdrawal means 64 preferably has means for regulatingthe anolyte liquid level 62. Anolyte liquor withdrawal means 64 is thuspreferably a pas'- sageway for liquids which is capable of being rotatedabout an axis passing through hole 66 through which anolyte withdrawalmeans 58 is attached. Thus, anolyte liquid level 62 can be changed toincrease or decrease the hydrostatic head within anolyte compartment 46,as may be preferable when increasing or decreasing the causticconcentration in the catholyte compartment. Handle 68 is provided to aidin rotating anolyte liquor withdrawal means 64 when adjusting theanolyte level 62.

It is obvious that numerous modifications can be used to provide anolytewithdrawal means from the anolyte compartment. Such modifications can beeither with or without anolyte liquid level adjustment means. An exampleof other adjustment means is an externally mounted inverted U tube whichcould be rotated to change the liquid level within the cell. Also, meanssimilar to those used in regulating the catholyte liquid level could beused. Such other modifications will be readily apparent to those skilledin the art from the description herein.

Anolyte liquor withdrawn from anolyte compartment 46 by means ofwithdrawal means 58 is passed into stack 70 for return to theresaturator. Stack 70 has transparent sight glass 72 therein whereby theanolyte eluent liquor can be observed.

The following examples illustrate certain preferred embodiments of thepresent invention. Unless otherwise indicated, all parts and percentagesused herein are by weight and all temperatures are in degreescentigrade.

EXAMPLE 1 A group of 23 Hooker type S-l cells was operated in the normalmethod by feeding brine to the anolyte compartments of each cell at afeed concentration of 310 grams per liter of NaCl. The brine feed was ata pH of 9, which is the normal brine pH of feed liquor. A decompositionvoltage of about 4 volts at about 12,000 amperes per cell was passedthrough the cells in the normal manner thereby producing gaseouschlorine at the anode and hy drogen and caustic soda (cell liquor) atthe cathode. The caustic soda was withdrawn from the catholytecompartment of each cell as cell liquor. The group of cells wascontinuously operated for several weeks, during which time the operatingconditions of the cells were noted. The brine feed rate during theperiod of operation averaged 2.7 liters per minute per cell whichcorresponds to the flow through the diaphragm of each cell. It was foundthat the average current eiciency of the cells for this period was 95.5percent and that the anolyte temperature within the cells varied fromcell to cell within the range of 92 degrees centigrade to 104 degreescentigrade, the average being about 95A degrees centigrade. The brinestrength Within the anolyte compartments averaged 260 grams per liter ofsodium chloride.

A similar group of 23 Hooker type S-1 cells was operated in accordancewith the present invention wherein 24 hour period under the notedconditions of operation. Table II illustrates the advantages to begained in a commercial size plant producing 200 tons per day of chlorinebased on six months of operation.

the cells were fed a brine solution in parallel, as shown in In Tables Iand II, Example 2 illustrates the results FIG. 1, at a rate in excess ofthe ow through the diaobtained under conventional cell operations.Examples 3 phragm and Where the excess anolyte liquor was Withthrough 8show the results obtained utilizing the anolyte drawii', as shown inFIG. 1, using the anolyte liquor withrecirculation method of the presentinvention. Example 3 drawal means illustrated in FIGS. 2 and 3. Again,these illustrates the cell operation without an HCl addition 'of cellswere fed with a brine solution containing 310 grams 10 Examples 4through 8 illustrate the cell operation with per liter of sodiumchloride but the feed rate was in- HCl additions of varying percentages.The percentages creased from the normal rate of about 2.7 liters perminof HCl addition noted are based by weight on the amount ute to about7.0 liters per minute, or about an average of chlorine produced by thecell. of 2.5 to 2.75 times the ow through the diaphragms of The celloperating temperatures averaged about 95 said cells. The excess anolyteliquor, which varied slightly degrees centigrade, varying from 94 to 96degrees centifrom cell to cell within the range of about 4 to 5 litersgrade, for the anolyte recirculation examples. The operaper minute, theaverage being about 4.3 liters per minute, ting temperatures of thestandard Hooker type S-3 cells was withdrawn from the cells, mixedtogether and realso averaged about 95 degrees centigrade, but theinsaturated to 310 grams per liter of NaCl prior to being dividual cellsvaried widely from about 92 degrees centireturned to the anolytecompartments of the cells. Under grade to about 100 degrees centigrade.these conditions, the sodium chloride concentration in the All of thecells were operated at 30,000 amperes. The anolyte compartment averaged295 to 300 grams per liter brine feed rate to the standard S-3 cellsaveraged about of sodium chloride. The pH of the anolyte liquor with-6.44 liters per minute. The brine feed rate to the anolyte drawn fromthe anolyte compartments averaged 3.5 to recirculation Examples 3through 8, averaged about 20 4.0. The resaturated and replenished brinereturned to liters per minute, with the amount of withdrawn anolyte thecell had a pH of 6.5. The temperature within the liquor ranging fromabout 13.5 to 17 liters per minute. anolyte compartments of the cellsvaried from 94 to 96 The lesser Withdrawal rate, i.e., about 13.5 litersper degrees centigrade, the average being 95 degrees centiminute, wasused in Examples 3 through 5 to correspond grade. After extendedoperation, it was found that the to a flow through the diaphragm aboutequal to that of group of cells using the anolyte recirculation methodof EXamPlC 2, and the greatest Withdrawal rate was used in the presentinvention had improved current eiciencies Examples 7 and S. The lesserflow rates through the diacompared to the cells operated in aconventional manner. phragm resulted in producing more concentratedcaustic The average current eiciency of the cells operating by in thecatholyte compartment. The particular flow rate the anolyterecirculation method was 96.4 percent. or tllle cells uedhin the presentmethod was regulated y c anging t e ydrostatic rhead in the anolyte com-EXAMPLES 2 8 partment by means of the apparatus of FIGS. 2 and 3. Themethod of the present invention was operated in Slight variations in thewithdrawal rate were noted due accordance with FIG. l and the methoddescribed in the to differences in diaphragm porosity. specificationwherein cells modified as shown in FIGS. 2 All of the cells were fedwith a nearly saturated brine and 3 were used, this method was comparedwith normal 40 containing about 315 grams per liter of sodium chloridecell operations as in a production size operation. The com- `at a feedtemperature of about degrees centigrade. The parison was made between agroup of 10 standard 30,000 anolyte sodium chloride concentration forthe cells of arnperes Hooker type S-3 cells which produce about oneExample 2 was about 270 grams per liter whereas the ton of chlorine percell per day and an equal number of cells of Examples 3 through 8 hadsodium chloride con- S-3 cells modified as in FIGS. 2 and 3, which wereop- 45 centrations between 280 and 310 grams per liter, `the erated inaccordance with the present invention. Table I average being about 300grams per liter. tabulates the average results obtained during anaverage Table I shows the results obtained as follows:

TABLE I Example Number Normal Anolyte Recireulation Cells Percent HC1Added 0 0 3 7 13 19 Cell Liquor, grams per liter of NaOH. 140 140 140170 210 260 Anolyte pH at 25 degrees centigrade.. 3.8 3.8 3.4 2.7 2.72.7 2.7 Anode Current Eniciency,percent 95.2 96.0 90.5 97.2 97.2 97.297.2 Cen Life, days 250 259 300 375 375 375 375 Typical Cell Voltage...4.00 3.99 3. 99 3. 99 4.00 4.01 4. 02

TABLE II.-DAILY PRODUCTION RATES FOR 200 TON PER DAY PLANT Method ofExample Number 2 3 4 5 6 7 8 Tons of salt used 331 331 328 321 310 290262 Tons of water to be evaporated to make 50 percent NaOH- 1, 369 1,369 1, 355 1, 331 918 641 488 Tons of fresh brine to be heated 2, 621 2,654 2, 588 2, 529 2, 020 1, 610 1,340

CHANGES DUE TO ANOLYTE RECIRCULATION Tons o1 NaOH not produced- 0 0 2 615 28 42 Tons ofsalt not used 0 0 3 10 21 41 69 Tons of water notevapolat rl 0 14 38 351 728 881 Tons of brine not heated. I) -33 33 92601 1, 011 1, 281

Percent current reduct 1on.. 0 84 1. 37 2. 1 2. 1 2. 1 2. 1

Percent voltage reduetlon U 25 25 25 0 25 50 Percent reductions in cellsrenewed 0 3. 5 16. 7 33. 3 33. 3 33. 3 33. 3

A comparative analysis of Example 2, which is the average ofconventional cell operation, with the examples of the anolyterecirculation method las illustrated in Examples 3 through 8, shows thateven without the addition eiciencies and the over-al1 effects of theanolyte recirculation method of the present invention. All of theexamples utilized a brine feed having a sodium chloride concentration of315 grams per liter. The cells were opof HC1 to control the anolyte pH,improved results are 5 erated at 12,000 amperes. Example 10 shows theoperaobtained in current reduction, voltage reduction and a tion of anormal cell without anolyte recirculation. Exreduction in the percentageof cells renewed. With the ample 11 illustrates the anolyterecirculation process of addition of HC1, increased anode efficiency isnoted in the present invention without acid addition, wherein theaddition to markedly improved cell life. The current rebrinerecirculation is at a rate of about 2.3 times that duction and reductionin percentage of cells renewed is lo flowing through the diaphragm whichcorresponds to a also markedly improved. By regulating the flow of brinefeed rate of about 6.2 liters per minute, 3.5 liters of through thediaphragm by raising or lowering the hydrowhich were recycled. Thewithdrawn anolyte solution is static head -in the anolyte compartment,the amount of resaturated and the temperature adjusted prior toreturnsodium hydroxide contained in the cell liquor can be also ing thebrine to the cell to provide'an anolyte temperature changed so as togreatly improve the caustic salt ratio in 15 of about 98 degreescentigrade. the catholyte cell liquor. A decrease in the ow rateExamp1es 12 through 14 employed the same recirculathrough the diaphragm,as by lowering the hydrostatic tion method as in Example 11 but with areduced hydrohead, increases the caustic concentration in the catholyteStatie head within the anolyte compartment to increase compartmentthereby increasing the demand for HC1 to the caustic concentration inthe catholyte compartment maintain the desired pH due to an increase inthe back by reducing the ow through the diaphragm. At a feed migrationof hydroxyl orlS through tho diaphragm Higher rate of 3.6 liters perminute about 1.6 liters per minute of caustic concentration greatlyreduces the amount of water anolyte liquor were Withdrawn from lheanolyte compart. to be evaporated to make 50 Percent Sodium hydroxidement for recirculation. The reduced hydrostatic head reand reduces :theamount of fresh feed brine to be heated. duced the ow through thediaphragm to about 2 liters The economic advantages of the process areclearly illusper minute, The feed brine was aeidied with HC1 to a tratedin Table II. pH of 0.2, the amount of HC1 added was chosen to provide Asis seen from an examination of Tables I and II, the the noted anolytepI-L Present method Carl be Used .to regulate the Proportion Examples 15through 17 were run in the same general of chlorine'to sodium hydroxldeproduced.l As .has often manner as Examples 12 through 14, but with theanolyte been the Sltuatlon the demand for Chlolfnfe 1S greater 30recirculation rate being about 1.5 times that flowing than that ofsodium hydrox1de..Therefore, 1t1 s often prefthrough the diaphragm intothe catholyte compartment erabie to produce .more chlorme than .caustlThls can This corresponded to a feed rate of about 3.3 liters perreadily be accomphshed by the present Invention' minute of brine ofwhich about 1.1 liters per minute of EXAMPLE 9 anolyte liquor werewithdrawn for recirculation, the re- This example illustrates thepreferred operating condima1r1mg 22 me Per minute Passed through thedla' tions of the present invention under varied conditions of Phragmmto the catholyte Compartment- Current. Calculated cell Anolyte pHAnolyte, Cell liquor, Exemple Efficiency, life-days based at 25 degrees.grams per grams per Number Percent on six months centigrade liter ofNaCl liter of NaOH ot operation 95. 2 250 3. 8 240 140 96. o 259 3. s30o 140 9e. o 375 2. 7 279 252 93. 9 (l) 4. 05 233 259 se. 3 375 2. 6280 245 96.2 375 2.7 248 21o 92.4 (1) 4.3 248 223 97l 2 (l) 2. 1 251 213l Not determined.

HC1 additions and cell liquor caustic concentration. FIG. 4 shows theaverage relationship between cell liquor strength, current eiciency andr-ate of HC1 addition for ch-lor-alkali diaphragm cells operated at anaverage anolyte temperature of 94 degrees centigrade and an anolyterecirculation rate sufficient to provide an anolyte NaCl concentrationof 300 grams -per liter at a feed concentration of 310 to 315 grams NaClper liter. The data given in FIG. 4 are the averages obtained fromnumerous runs using Hooker type S-l production cells operated inaccordance with the invention at 12,000 amperes. Curve A illustrates therelationship between HC1 additions and the amount of caustic which isretained in the cell liquor to obtain an anolyte pH at a level whichwill provide a 98 percent current eiciency. Changes in this relationshipbring labout corresponding changes in current eciencies as isillustrated by curves B, C and D.

The illustrated curves are for constant conditions of anolytetemperature and anolyte sodium chloride concentrations. Changes in theseconditions will displace or alter the slopes of the curves.

EXAMPLES 10-17 Individual Hooker type S-l cells were operated inaccordance with the present invention to determine the effects of highcatholyte caustic concentrations on cell ab out 4.

While there have been described various embodiments of the presentinvention, the methods described are not intended to be understood aslimiting the scope of the.

invention. It is realized that changes therein are possible and it isfurther intended that each ele-ment recited in any of the followingclaims is to be understood as referring to all equivalent elements foraccomplishing substantially the same results in substantially the sameor equivalent manner. It is intended to cover the invention broadly inWhatever for-m its principles may be utilized.

We clai-m:

1. A process -for operating a group of chlor-alkali diaphragm cellscomprising imposing a decomposition voltage across the electrodes ofsaid chlor-alkali diaphragm cells, feeding a solution of brine to theanolyte compartment of each of said cells at a rate in excess of theamount which flows through the diaphragm of said cells, Withdrawing theexcess brine feed solution from the anolyte compartments of said cells,combining and replenishing the withdrawn solutions with additional:amounts of a chloride selected from the group consisting of an alkalimetal chloride, hydrogen chloride and mixtures thereof,

and returning the replenished brine solution to the anolyte compartmentsof said cells.

2. The process of claim 1 wherein the temperature of the brine feedsolution is adjusted to obtain an anolyte temperature of about 85 to 100degrees centigrade.

3. The process of claim 1 wherein the brine feed solution fed to thecell contains 130l to 330 grams per liter of sodium chloride.

4. The process of claim 1 wherein the brine fed to the anolytecompartment is nearly saturated sodium chloride solution.

5. T he process of claim 4 wherein the brine feed solution is fed to theanolyte compartment at a rate of 1.5 to about times the rate at -whichthe brine passes through the diaphragm into the catholyte compartment ofthe cell.

6. The process of claim 1 wherein acid is Iadded to the brine feedsolution in an amount to obtain an anolyte pH within the range of about1.5 to 4.0i.

7. The process of claim 6 wherein HCl is added to the brine feedsolution in an amount to obtain an anolyte pH of about 2 to 3.

8. The process of claim 6 wherein the brine feed solution is aciditiedby the addition of up to about i weight percent HCl based on the amountof chlorine produced by the cell.

9. A process for operating a Chlor-alkali diaphragm cell comprisingfeeding a solution of brine to the anolyte compartment of theChlor-alkali diaphragm cell while imposing a decomposition voltageacross the electrodes of said cell, feeding said brine solution at arate of about 1.5 to 10 times the amount which flows through thediaphragm of the cell into the catholyte compartment of the cell,withdrawing the excess brine feed solution from the anolyte compartmentof the cell, replenishing the withdrawn solution with salt, adjustingthe temperature of the solution to the desired feed temperature `andreturning the resaturated and adjusted brine solution to the anolytecompartment of the cell.

10. The process of claim 9 wherein the temperature of the brine feedsolution is adjusted so as to obtain an anolyte temperature of about 93to 100 degrees centi grade.

11. The process of claim 9 wherein the brine fed to the anolytecompartment contains about 280 to 330 grams per liter of NaCl and therate of feed is sufficient to maintain an anolyte sodium chlorideconcentration in the anolyte compartment of about 280 to 330` grams perliter. t

12. The process of claim 9 wherein the pH of the anolyte solution -inthe cell is maintained between about 1.5 and 4 by the addition of up to:about 20 weight percent hydrogen chloride based on the amount ofchlorine produced in the cell.

13. The process of claim 9 wherein the amount of anolyte feed solutionpassing through the diaphragm into the catholyte compartment isregulated by |changing the level to which the anolyte liquor iswithdrawn by adjustment of the anolyte liquor withdrawal means therebychanging the hydrostatic head within the anolyte compartrnent.

14. The process of claim 9 wherein the brine feed solution fed to thecell contains about 130 to 330 grams per liter of sodium chloride.

15. The process of claim 9 wherein the brine feed solution is a nearlysaturated sodium chloride solution.

16. The process of claim 15 wherein said brine solution is fed to thecell at a rate of about 2 to about 5 times the amount which llowsthrough the diaphragm of the cell into the catholyte compartment, theexcess brine feed solution is withdrawn from the anolyte compartment andresaturated with salt, the temperature of the resaturated solution isadjusted to obtain an anolyte temperature of about '93 to 100 degreescentigrade, the acidity of the brine feed solution is adjusted by theaddition of hydrogen chloride to obtain an anolyte pH of about 2 toabout 3 and the resaturated and adjusted brine solution is returned tothe anolyte compartment of the cell.

17. The process of claim 16 wherein the amount of HCl added to the brinefeed solution is in an amount of up to about 2() weight percent HClbased on the weight of chlorine -produced by the cell, said HCl additionbeing in an amount commensurable with the back migration of the hydroxylion from the catholyte compartment to provide a pH in the anolytecompartment of about 2 to 3.

18. The process of claim 17 `wherein the caustic concentration in thecatholyte compartment of the cell is increased by lowering thehydrostatic head of anolyte liquor within the anolyte compartment andincreasing the `amount of HCl added to the brine to provide an anolytepH of about 2 to 3.

19. A process for Iincreasing the caustic concentration of cell liquorin a Chlor-alkali diaphragm cell comprising `feeding t-o the anolytecompartment of said chlor-alkali cell a nearly saturated solution ofbrine while imposing a decomposition voltage across the electrodes ofsaid cell, said brine feed solution being acidied with HC1, feeding saidsolution at a rate in excess of the amount which flows through thediaphragm into the catholyte compartment of the cell, adjusting thehydrostatic head of the anolyte liquor to lessen the flow rate throughthe diaphragm of said cell, regulating the HCl acidification of thebrine to produce an anolyte pH below about 4, withdrawing the excessbrine feed solution from the anolyte compartment, resaturating saidwithdrawn solution with salt and returning the resaturated brinesolution to the anolyte compartment of the cell.

20. The process of claim 19 wherein the hydrostatic ,head within theanolyte compartment of the cell is redu-ced to lessen the ilo-w throughthe diaphragm and thereby increase the concentration of caustic withinthe catholyte compartment.

References Cited UNITED STATES PATENTS 2,628,935 2/1953 Earnest et al.204--95 2,925,371 2/196() Winckel et al. 204--239 XR 2,954,333 9/ 1960Heiskell et al 204--98 3,043,757 7/1962 Holmes 204-95 3,052,612 9/1962Henegar et al. 204--128 3,055,821 9/ 1962 Holmes et al. 2011-270 HOWARDS. WILLIAMS, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

1. A PROCESS FOR OPERATING A GROUP OF CHLOR-ALKALI DIAPHRAGM CELLSCOMPRISING IMPOSING A DECOMPOSITION VOLTAGE ACROSS THE ELECTRODES OFSAID CHLOR-ALKALI DIAPHRAGM CELLS, FEEDING A SOLUTION OF BRINE TO THEANOLYTE COMPARTMENT OF EACH OF SAID CELLS AT A RATE IN EXCESS OF THEAMOUNT WHICH FLOWS THROUGH THE DIAPHRAGM OF SAID CELLS, WITHDRAWING THEEXCESS BRINE FEED SOLUTION FROM THE ANOLYTE COMPARTMENTS OF SAID CELLS,COMBINING AND REPLENISHING